But cases of introduction such as those above referred to are by no means confined to the vertebrates, similar instances among invertebrates being numerous enough. I am sure every naturalist is personally acquainted with a good number, and it is hardly necessary that I should quote in any detail after what has been said on the subject generally. The two species of snails, _Helix pomatia_ and _Cyclostoma elegans_, both of which occur in England, and which I had occasion to mention among those experimented on by Darwin, were turned out in several suitable localities in Ireland by Thompson, but failed to establish themselves. The former, according to Mr. Kew, was also introduced into Scotland and Norway, whilst fifty or sixty specimens were brought to Petersfield in England, but none of these trials at acclimatisation were successful. As among vertebrates, a large number of the so-called successful introductions rest upon insufficient evidence. When we once more carefully review the evidence as to the undoubted difficulty attendant on intentional introduction of animals by human agency, placed as they often were in most suitable localities, we must feel that accidental introduction cannot play an important rôle in the making of the fauna of any country. Especially is this the case with an island fauna. Vertebrates are almost altogether excluded, and invertebrates must arrive singly as a rule, often stranded on an inhospitable and unsuitable shore. Their chances of surviving a passage by sea, of finding suitable food and shelter and a mate in order to procreate their species, appear to me infinitesimally small. Yet there may be some such cases. However, I quite agree with Mr. Andrew Murray--a high authority on geographical distribution--that "colonisation or occasional dispersal is insufficient to account for the character of the faunas and floras of oceanic islands; and I believe that the normal mode in which islands have been peopled, has been by direct continuity with the land at some former period, or by contiguity so close as to be equivalent to junction" (p. 15). "That a slight intermixture," he continues, "due to Mr. Darwin's colonisation, occurs in many (probably in all) I am ready to admit; and from instances to be afterwards noticed, I am disposed to reckon the proportions of such intermixtures in the flora, in the most favourable circumstances, at not more than two per cent. In the fauna I think it must be much less."